The present invention relates to an exhaust gas cleaner and a method of cleaning an exhaust gas by using this exhaust gas cleaner.
Recently, much attention has been being paid to fine particle materials contained in exhaust gases discharged from diesel engines, etc. particularly fine solid carbon particles and fine particles of liquid or solid high-molecular weight hydrocarbons, which cause environmental pollution. These fine particles, which may be called simply "fine carbon particles," have an average particle size of about 0.1-1 .mu.m. Accordingly, they are floating in the air, and easily inhaled by breathing. It is confirmed by recent clinical tests that they contain carcinogens.
There are essentially two methods of removing these fine particles. One of them is a method of filtering an exhaust gas with a heat-resistant filter to trap fine carbon particles, and burning the fine carbon particles by a burner, an electric heater, etc. after a large pressure loss appears, to reuse the filter. Such filters may be metal wire meshes, ceramic foams, honeycomb ceramic monoliths, etc. The other is a method of using a heat-resistant filter carrying catalytic materials to filter and burn fine carbon particles, making the burning of fine carbon particles to regenerate the filter less frequent or making the regeneration of the filter unnecessary, thereby increasing the burning activity of the catalyst.
In addition, an attempt has been made to lower an ignition temperature of the fine carbon particles in these methods by increasing combustible components in the exhaust gas or by adding agents for promoting the oxidation of fine carbon particles to a fuel oil.
In the former method, however, the higher the efficiency of removing fine carbon particles, the faster the pressure loss takes place, resulting in more frequent regeneration steps. Thus, high level of regeneration is required, which means that it is economically disadvantageous.
On the other hand, in the latter method, it would be considered an excellent method, if there is a catalyst capable of maintaining a catalytic activity under the conditions of exhaust gases from diesel engines (gas compositions and temperatures). However, the exhaust gases from diesel engines have much lower temperatures than those from gasoline engines, and because the diesel engines use a gas oil as a fuel, their exhaust gases contain a large amount of SO.sub.2. There has been proposed no method of burning fine carbon particles accumulated on catalysts under such exhaust gas conditions and regenerating them without causing any secondary pollution.
For instance, a combustion reaction of fine carbon particles can take place on a heat-resistant filter carrying only base metals at a temperature of about 350.degree. C. or more. During most of the diesel engine operations, however, the exhaust gas temperatures are lower than the above temperature, causing no combustion reaction. As a result, fine carbon particles are trapped on the catalyst-carrying filter, and after the temperature increases over the above level, they start burning, enabling the regeneration of the filter. In this case, until the temperature of the filter reaches the above level, the pressure loss increases largely, making it more frequent to regenerate the filter by burning it with a burner, an electric heater, etc. Thus, this method is not practical one.
Alternatively, in the case of a catalyst containing precious metals, a combustion reaction of fine carbon particles easily takes place at a lower temperature, making slower the increase in the pressure loss. However, since the oxidation of SO.sub.2 in the exhaust gas takes place simultaneously, extremely poisonous SO.sub.3 or sulfuric acid mists are also generated, causing secondary pollution.
As described above, a good method has never been found to remove the fine carbon particles from the exhaust gases of diesel engines, etc. efficiently without oxidizing SO.sub.2 to SO.sub.3.
Another culprit of environmental pollution discharged form engines, etc. is nitrogen oxide simply represented by NO.sub.x. NO.sub.x is usually removed from exhaust gases of engines, etc. by reducing it to N.sub.2 with proper catalysts. However, in the case of exhaust gases from diesel engines, it is extremely difficult to remove nitrogen oxides, because exhaust gas temperature is relatively low, and because their oxygen concentration changes widely from several % to 13% or so.